Separate the "{img ...}" remarkup rule into separate parse and markup phases

Summary:
Ref T13101. Ref T4190. This rule is currently single-phase but I'd like to check for a valid proxied image in cache already and just emit an <img ... /> tag pointing at it if we have one.

To support batching these lookups, split the rule into a parse phase (where we extract URIs) and a markup phase (where we build tags).

Test Plan: Used {img ...} in Remarkup with no apparent behavioral changes. (This change should do nothing on its own.)

Maniphest Tasks: T13101, T4190

Differential Revision: https://secure.phabricator.com/D19192